Mobile networks are generally capable of providing information on the whereabouts of their subscribers, e.g. to an emergency facility, a traffic surveillance centre or other service unit needing or even requiring such positioning information. In general, mobile networks may sometimes be required to provide and certify the location or position of a subscriber in order to support emergency services and other location dependent services. Various positioning functions are therefore typically employed in the mobile networks for locating terminals connected to base stations in cells of the network.
These positioning functions may include simply identifying the cell currently serving a terminal of interest, which can provide an accurate enough position when the terminal is connected to a base station serving a relatively small cell, but not particularly accurate when connected to a base station serving a larger cell. A more accurate position may further be derived from a used timing advance when the serving cell is known and/or signal strength measurements on signals from different base station sites, the latter method being known as “triangulation”.
The concept of time alignment or timing advance is generally used in mobile networks employing time division multiplexing where terminals sharing the same transmit frequency are directed to transmit their signals during allocated timeslots, commonly referred to as TDMA (Time Division Multiplex Access). FIG. 1a illustrates schematically how timing advance is used in a cell covered by a base station BS. Three mobile terminals A, B and C are currently connected to the serving base station BS, and different timeslots 100 are allocated to the terminals such that terminals A, B, C are directed to transmit signals “A”, “B” and “C” in successive timeslots 100, respectively, as indicated in the figure. The terminals A-C are thus synchronised with BS to allow for proper timing of the transmission and reception of signals.
In this example, terminals A and C are located relatively close to BS, while terminal B is located at a greater distance from BS. As a result, the signals from terminals A and C will arrive basically “in time” to BS while the signals from terminal B would arrive somewhat late due to propagation delays, thus not exactly fitting into the allocated timeslot when received at BS, which could cause interference due to overlap with signals from terminal C in this case. In order to avoid such interference, BS orders terminal B to transmit its signals somewhat earlier by a parameter called Timing Advance TA. This mechanism is generally referred to as time alignment. Thus, by adjusting the timing of transmissions from terminal B in this way, the signals will arrive properly at BS in the allocated timeslot as indicated in the figure.
Although the parameter TA was originally conceived to adjust terminal transmissions to fit into a timeslot scheme at the receiving base station, TA has been frequently utilised to provide location information as well. As the propagation speed of radio signals is known to equal the speed of light C, the TA used by a specific terminal further implies the distance D between that terminal and the serving base station as D=C×TA. According to 3GPP (3rd Generation Partnership Project), TA is specified as an integer between 0 and 63 representing time steps in the interval 0 μs through 232 μs, each step thus representing approximately 3.7 μs which corresponds to 553 m of signal propagation. The location of a terminal can thus be estimated by knowing the location of the serving base station and the TA used. In many mobile systems, the base station's location is basically given by a parameter “CGI” (Cell Global Identity) providing the coordinates of the base station.
FIG. 1b illustrates that when a mobile terminal, not shown, is directed by a serving base station BS to use a specific timing advance value TA to adjust its transmissions, that TA value can further be used to calculate an expected terminal distance from BS as being within a potential position area P at a distance of TA×553 m from BS, according to 3GPP. If BS covers 360°, i.e. an omni cell, the terminal is presumably located somewhere within a circle area or ring P(Omni), while if BS covers a sector less than 360°, i.e. a sector cell, the terminal can be somewhere within a circle sector area P(Sector), as illustrated in the figure. For example, if TA=10, the terminal is expected to be located at a distance of around 5.5 km from the base station. Since TA is specified in 3GPP according to predefined integers, the expected terminal/base station distance can be determined within an uncertainty interval of 553 m.
The above positioning method is frequently used in GSM systems and other similar mobile systems using time alignment. Utilising the CGI/TA information for positioning is particularly attractive since it is promptly available at the serving base station or at a base station controller BSC, and no further measurements nor added functionality in terminals are necessary.
However, the CGI/TA based positioning method is sometimes not very accurate as the TA parameter is primarily determined to make terminal transmissions arrive in time for allocated timeslots at the base station, but not as a measure of distance. At present, mobile systems are not obliged to set the TA parameter for correct position determination. As a result, differently configured base stations, e.g. made by different vendors or operators, may consistently set different TA values for terminals located at the same distance, yet providing equivalent communication quality. This trait has actually been confirmed by field testing.
Using TA for terminal transmissions not only compensates for propagation of radio waves in the air but also for any delays due to propagation, switching and processing of signals within the base station, which may differ depending on the configuration of hardware and software in the base station. It is thus a problem that the CGI/TA based positioning method is not wholly reliable and may produce different results depending on base station configuration. It would be desirable to produce basically the same positioning results regardless of the equipment used.